The present invention relates to a linear actuator and linear position sensing apparatus for detecting the position of the linear actuator such as, for example, a hydraulic or pneumatic cylinder.
In many applications where hydraulic or pneumatic cylinder actuators are used to control the movement or positioning of an object it is often desirable to determine the displacement of the actuator.
A typical hydraulic or pneumatic piston actuator comprises a cylinder that houses a slidable piston and piston rod assembly arranged for reciprocal movement in the axial direction. The piston is sealed to the inside surface of the cylinder so as to divide the cylinder into two chambers and is moveable, under the influence of hydraulic or pneumatic fluid introduced under pressure into one or other of the chambers, between a retracted stroke position in which the piston rod is substantially wholly received within the housing and an extended stroke position in which the length of the rod projects out of the housing. The movement of the piston is typically effected by using one or more control valves to introduce the fluid into the chambers. In order to ensure accurate positioning it is desirable to operate the control valves in response to a feedback signal representing the position of the piston or piston rod relative to the cylinder in which case it is necessary to have the ability to sense the stroke position of the piston or piston rod in an accurate manner.
The conventional approach to incorporating a position sensor in a linear actuator of this kind is to drill a bore along the longitudinal axis of the piston rod into which at least part of a sensor arrangement can be fitted. One example of such a sensor is a linear voltage displacement transducer. Another is a magnetostrictive transducer comprising an elongate waveguide disposed in the bore and a magnet arranged around the piston rod such that its magnetic field is directed along the waveguide. Current pulses are sent from a sensor fixed in the cylinder and propagate along the waveguide. The magnetic field generated by each pulse interacts with the magnetic field of the magnet such that a mechanical strain is imparted in the waveguide. This strain is sensed and converted into an electrical pulse and the position of the magnet relative to the waveguide can be determined from the time taken for the pulse to travel the distance between the magnet and the sensor.
In another example a series of Hall-effect sensors or reeds are arranged in linear array in a tube along the bore in the piston rod and a permanent magnet fitted to the piston rod slides relative to the tube thus activating each of the sensors in turn.
The machining of a bore in the piston rod to accommodate part of the sensor assembly is undesirable as it increases the manufacturing cost and potentially weakens the actuator. This is particularly a problem with long stroke cylinder actuators.
An alternative approach is to use a sensor external to the cylinder and a magnet with pole pieces attached to the piston. This involves adapting the piston in such a manner that additional components increase its length resulting in either a reduced actuator stroke or the need to extend the length of the cylinder both which incurs undesirable additional manufacturing costs. It has also been realised that the exposure of the magnet and/or pole pieces to the end forces applied to the piston by high pressure within the cylinder can affect the integrity of the magnets which in turn affects the accuracy of the readings.
External sensors are often impractical as the actuators are used in harsh environments. Moreover, many hydraulic linear actuators are operated under significant pressure and so the cylinder tends to be made from thick steel. This renders the use of magnetic-based sensors problematic as the ferromagnetic properties of the thick steel cylinder means that the magnetic flux generated by the magnet is generally shielded from the external sensor and is generally not of sufficient density such that it can be sensed accurately.